Read BUILD files in bp2build

The parsed BUILD files will be scanned for obvious definitions of BUILD
targets which have Android.bp counterparts. In such cases, bp2build will
automatically omit conversion of these defined modules (to prevent
collisions). With this change, we no longer need one-off denylisting of
modules which have BUILD file definitions.

This has a 0.03s to 0.2s slowdown for bp2build with current state. This
impact is identical on a heavier test branch, as well. I also ran an
experiment that applied BUILD scanning to all source BUILD files
(regardless of allowlisting), and this had a 2 second slowdown.

We may want to look into parallelizing or improving the performance of
this evaluation, but it's probably not worth the effort at this time,
since the current performance hit is small.

Test: New integration test
Test: Removed libprotobuf-python from denylist and tested building the
package
Test: Treehugger

Change-Id: Ibde3bab12cd4a8fed642ad46e5344a56953bec91